For the first time in a generation, federal criminal justice reform seems more like inevitability than afterthought in Washington. With bipartisan legislation being considered in Congress, and President Obama taking full advantage of the bully pulpit to press for justice reform, it seems increasingly likely that a bill with the explicit aim of reducing the federal prison population will be enacted before the next election.

The bills now under consideration would reduce the most severe mandatory minimums for nonviolent drug offenses and expand earned-time credits for participation in risk-reduction programming. These are important steps toward reversing decades of federal prison population growth, but are they enough?

On Tuesday, the Urban Institute and the Charles Colson Task Force released the federal version of the Prison Population Forecaster, which allows users to simulate the impact of changes to prison admissions and length of stay. The forecasting tool makes it crystal clear that a dramatic reduction in the size of the federal prison population will require reducing time served and admissions for drug trafficking offenses, which is the primary conviction for nearly half the BOP population.

Changing prosecutor behavior is key to reducing the federal prison population

Reforms that reduce admissions and length of stay must focus on the role of federal prosecutors and the ways in which they use mandatory minimums to secure convictions and historically long prison sentences. While Congress may have intended these harsh punishments for only the most serious cases, mandatory sentences have become the tools of the trade for federal prosecutors. They alone have discretion over whether to apply those charges, and data suggest they reliably do in most district courts.

In fact, many people convicted at the federal level could be diverted to alternatives in the federal system, prosecuted at the state level (where sentences are much shorter), or sentenced to significantly shorter federal terms. Consider that one in three people in federal prison for a drug offense was in the lowest criminal history category at sentencing and nearly half of those sentenced at the federal level are street-level dealers, couriers, or mules, not the kingpins they’ve been made out to be. We’ve widened the net and we’re catching increasingly small fish—there are 50,000 more people convicted of drug crimes in federal prison today than two decades ago.

Will current bills make a difference?

The Sentencing Reform and Corrections Act (SRCA), the Senate bill given the best chance by political prognosticators, is a step in the right direction, but mostly focuses on reducing the most egregiously long sentences rather than eliminating the tools that prosecutors use to send people to prison in the first place or providing them with meaningful alternatives. And, as the forecaster shows, significantly reducing the federal prison population by shortening length of stay alone would require much deeper reforms to mandatory minimum sentences.

The SAFE Justice Act, introduced in the House, goes a step further than the SRCA and more directly addresses federal prison admissions by eliminating federal criminal penalties for simple drug possession in state jurisdictions, making probation presumptive for first-time, nonviolent offenses (and directing the U.S. Sentencing Commission to revise the sentencing guidelines accordingly), expanding problem-solving courts, and requiring the use of graduated sanctions for violations of supervision. Since it also reforms mandatory minimum sentences, its impact would likely be much greater.

Both pieces of legislation would eliminate some of the most punitive tools in the federal prosecutor’s toolbox. And prosecutors do change their behavior in response to shifting legislative priorities. Since Congress passed the Fair Sentencing Act in 2010, federal convictions for crack-cocaine have plummeted by more than half. So either there was a dramatic reduction in the manufacture, sale, and use of crack-cocaine, or prosecutors stopped pursuing those cases because they were no longer able to hang a long prison sentence over the head of defendants.

Using executive action to set federal prosecution priorities

Changing federal law is important, but not the only way to change prosecutor behavior. Unlike in the states, the president and the Attorney General don’t need to wait for legislation to recalibrate federal prosecution priorities. Federal prosecutors operate under the guidance of the Department of Justice and their decisions are constrained by department priorities. Since former Attorney General Eric Holder announced that federal prosecutors should no longer pursue mandatory minimums in less serious drug cases, there has been a significant reduction in both the total number of drug convictions and the number of convictions carrying a mandatory minimum term.

The federal prison population forecaster demonstrates that dramatic cuts to the BOP population will require reforms that affect both admissions and length of stay for drug trafficking. Accomplishing that goal is going to take a combination of ambitious legislative and executive changes that limit prosecutorial discretion and return some power to judges and other decisionmakers in the criminal justice system.

A guard looks on at right as prisoners move through the state prison Thursday, March 3, 2011 in Jackson, Ga. Photo by David Goldman/AP